1. Field of the Invention
The invention relates to a process for the preparation of mixed salts of 2,2,'-oxydisuccinic acid and carboxymethyloxysuccinic acid. The mixed salts of 2,2,'-oxydisuccinic acid and carboxymethyloxysuccinic acid are effective sequestering agents and are useful as builders in detergent compositions for household, institutional and industrial use.
2. Related Art
2,2,'-oxydisuccinic acid (ODS), carboxymethyloxysuccinic acid (CMOS) and their salts are known and are known to have utility as sequestering agents and detergent builders. A disadvantage of ODS and salts thereof as detergent builders is that they are relatively expensive to prepare.
U.S. Pat. No. 3,128,287 to Berg, discloses a preparation of ODS salt by admixing maleic acid with an excess of hydroxide of calcium, barium, magnesium or strontium in the presence of water, then heating the reaction mixture from about one day to about one month at temperatures ranging from 50.degree. C. to reflux temperatures. The process yields a mixture of malic acid and ODS. Example I teaches the preparation of ODS, wherein the aqueous mixture of maleic anhydride and calcium hydroxide is reacted at reflux (100.degree. C.) for 4 days. Subsequently, ODS salt is isolated from the product containing ODS and malic acids salts.
The preparation of CMOS and salts thereof is described in U.S. Pat. Nos. 3,692,685 and 3,914,297 to Lamberti et al. The reaction involves a Michael type addition of glycolic acid to maleic acid in the presence of calcium hydroxide. U. S. Pat. No. 3,692,685 also discloses CMOS and its salts as detergent builders.
U.S. Pat. No. 3,635,830 to Lamberti et al., discloses the process for the preparation of ODS based on the process of Berg. The patent teaches separation/purification of two diastereoisomeric forms of ODS obtained by the Berg process. The patent also discloses detergent compositions with ODS or its salts as detergent builders.
U.S. Pat. No. 4,798,907 to MacBriar et al. discloses an improvement in the ODS-forming processes of Berg and Lamberti et al., wherein an alkali metal hydroxide, e.g. sodium hydroxide, is incorporated into a starting reaction mixture. To produce yields of about 80% ODS the process generally involves reacting the mixed starting materials in water for at least 12 hours at temperatures of about 20-100.degree. C.
European Patent 236,007 to Bush teaches a process for the preparation of ODS similar to the process of the MacBriar patent. Mixtures of inorganic base with water-soluble, inorganic salts of sodium, calcium or mixtures thereof may be used. A particularly preferred reactant combination constitutes maleic acid, malic acid, calcium hydroxide and sodium hydroxide. An ODS yield of about 60% is obtained in about 6 hours.
The MacBriar et al. and Bush patents rely on incorporation of alkali metal hydroxide into the reactant mixture to minimize gelation of the reactants and to increase yields of ODS. Alkali metal hydroxides indeed produce homogeneous solutions, but unlike the alkaline earth metal hydroxides, generate higher pHs and result in a gradual decomposition of ODS to fumarate and malate under the reaction conditions employed by the MacBriar et al. and Bush patents.
The patents referred to above share the same disadvantages: excessive energy requirements or lowered yields of ODS due to gelation of the reaction medium and the presence of unreacted starting materials or side products. There have been different approaches to the problem of increasing the yields of ODS while lowering the production cost and avoiding gelation. However, none of these approaches has been completely satisfactory.
Accordingly, it is an object of the present invention to provide a process for the preparation of the mixed salts of 2,2'-oxydisuccinic acid and carboxymethyloxysuccinic acid (ODS/CMOS). The process comprises forming ODS salt and utilising unreacted starting materials to form CMOS salt. Thus, the amount of unreacted starting materials is decreased and the yield of a usable builder product is increased. The mixed ODS/CMOS salts can be used as a detergent builder or a sequestering agent.
It is a further object of the present invention to provide a process which produces the mixed ODS/CMOS salts and substantially minimizes the gelation of a reaction mixture.
These and other objects and advantages will appear as the description proceeds.